1. Field of the Invention
The present invention concerns a technique for detecting the servo data in a hard disk drive, and more particularly to a technique for decoding gray code data in a hard disk drive (HDD).
2. Description of the Related Art
Generally, a disk drive is used with either floppy or hard disks. Each disk surface is formatted by being divided concentrically into a number of circular recording tracks each of which is in turn subdivided into servo and data sectors. Servo sectors are areas on which servo information for reading and writing data onto object tracks is recorded according to corresponding servo patterns, and data sectors are areas on which data transferred from the host computer are recorded. A servo address mark (hereinafter sometimes referred to as a "SAM") representing the synchronization point of time is recorded on the start point of a servo sector. Usually, a unique pattern which is not used in servo and data sectors, is recorded in the servo address mark area. The servo address mark area is followed by a gray code area containing ID information for each of the tracks. The gray code area has the characteristic that the patterns are easily distinguished from each other and identified by changing pattern codes sequentially by only one bit. Lastly, a servo sector has a burst area which contains the "on track" information used for positioning the read/write head on the object track during track following or seek. The gray code area of the servo area is arranged in a specific format, which is, in turn, subdivided into a gray-sync area for providing sync signals, and a gray-data area with actual track information recorded thereon.
An earlier gray code detecting method is as follows. At first, the servo data recorded in the servo address mark and the gray code areas within the servo area are retrieved from the read/write channel to be digitized, and the digitized signal is supplied to the gray code decoding circuit. At this time, the gray code decoding circuit generates first a gray enable signal, and then a decoding edge signal synchronized with the positive going pulse edge of the gray code signal. Further being based on the gray enable signal the gray code decoding circuit generates a gray synchronization detection window signal and gray data detection window signals. Accordingly, when the falling edges of the decoding edge signals are detected within the respective detection windows, the gray synchronization and the gray data signals are read out.
The patent to Schowe, U.S. Pat. No. 5,396,240, entitled Missing Pulse Generator For Gray Code Decoding, is directed to different techniques, while the patents to Silva and Setoyama, U.S. Pat. Nos. 5,115,356 and 5,546,243, respectively entitled Decoder Circuit With Missing Clock Generator, and Data And Synchronization Signal Outputting Apparatus For Recovering Missing Data And Synchronization Signals, each disclose circuits for generating missing clock signals.
Other exemplars of the state of the art include U.S. Pat. No. 5,353,175 to Chiba, entitled Apparatus For Signal Generators To Enable Detection Of Header Information Such As An Address Mark Sector Mark Or Address Information Recorded On Each Sector Of A Disc Recording Medium Only Where Such Information Could Be Present, U.S. Pat. No. 5,416,760 to Masood et al., entitled Recovery Of Data From Optical Data Disk Sectors Having Missing Or Defective Synchronization Information, U.S. Pat. No. 5,436,770 to Muto et al., entitled Disc Recording And/Or Reproducing Apparatus Having An Enlarged Locking Range For Correctly Phased Internal Channel Clocks, U.S. Pat. No. 5,001,479 to Becker et al., entitled Gray Code Converter With Error Signal, U.S. Pat. No. 5,381,281 to Shrinkle et al., entitled Disk Drive System Using Multiple Embedded Quadrature Servo Fields, U.S. Pat. No. 5,617,536 to Freitas et al., entitled Method And Apparatus For Servo Control With Error Detection Of Gray (Pennington) Code Used For Servo Track ID, U.S. Pat. No. 5,418,657 to Machado et al., entitled Track Address Detecting Means By HDD Sector Servo Method, and U.S. Pat. No. 4,189,757 to Kunstadt, entitled System For Decoding Data On A Magnetic Medium.
I have found that when supplying the signals retrieved via the read/write channel to the gray code decoding circuit to be decoded, the above retrieved signals are very weak owing to the flying height of the head or noise, resulting in being often missed. Consequently, erroneous decoding edge signals are generated and accordingly, the falling edge of the decoding edge signal can not be detected within the gray sync and data detection window, resulting in a failure to read out the gray sync and data. This is a substantial cause of erroneous decoding of the gray code decoding circuit and is a drawback of the earlier gray code detecting method.